Investment casting



Sept. 6, 1955 E. L. SCHELLENS 2,716,791

INVESTMENT CASTING Filed April 25, 1951 rfl g l I A 5047/5 Ore/2 36' United States Patent INVESTMENT CASTING Eugene L. Schellens, Essex, Conn.

Application April 23, 1951, Serial No. 222,491

Claims. (Cl. 22-200) parts weighing but a fraction of a pound; and as a typical instance the particular main disclosure herein pertains to certain turbine engine wear parts, namely, turbo-jet engine buckets, the intricate designs of which are very diflicult or even impractical of manufacture otherwise than by investment casting.

The system and other features constituting the invention fall, in part, within the general classified subject of metal founding, and in part, within other and collateral subjects. The subjects involved cover a field of important industrial scope, and the system of this invention is intended to meet special requirements believed not to have been heretofore satisfactorily answered.

The instance chosen for illustration is a typical use of the investment casting system for the production of such small metallic parts as those mentioned; including especially such articles as involve exposure to super-temperaures, the metal used having high heat resistant properties. The system of the present invention will be described in connection with the subject of turbine bucket production as a disclosed example, article or part, being an improvement in investment casting and shown as adapted to this typical purpose, and others.

The basic plan for the investment casting of metal products has been known in different forms for a number of years. Rather than setting forth some of the well known embodiments, and the shortcomings thereof, it is here pointed out that to the best of this applicants knowledge the art has remained underdeveloped, with substantially no recent salient progress, and has not been known in such form as to be entirely satisfactory, especially for more difficult work such as that for which the present invention was particularly devised.

Serious objections and difliculties have been encountered, constituting problems to be solved; and the main object of the present invention is to overcome the drawbacks encountered in prior and prevailing states of development, namely, by substantial improvement in the investment casting method and as well in the apparatus for carrying out such method, and the articles and other products and parts thus produced.

A particular object of the invention is the general improvement of known processes, not merely for general purposes, but more especially for the production of articles or parts of high quality and precision and adapted to withstand for prolonged periods the impairment to which such parts are exposed under destructive conditions, such as extremely high temperatures, mechanical strains etc. In the case of turbine buckets for jet engines Patented Sept. 6, 1955 ICC critical problems in production and use have been encountered and the more particular purpose of the present invention is so to improve the method and product as to meet these special problems, and to make the most of the virtues of the investment casting principle.

Other objects of the invention include the following: to provide a system in which the quantity production of precision products can be had with insured accuracy in reproduction of a desired design, the products being closely identical with previously prepared pattern bodies to be converted thereto. A further object is to simplify the prevailing system in various ways, including a timesaving reduction in the procedure and in the number of steps included therein.

In a specific aspect it is an object, and the present invention possesses the high advantage, among others, of dispensing with the heretofore prevailing step of first melting of the selected part metal, and then pouring it into the cavity of a previously prepared investment mold.

Other and further objects and advantages of the invention will be disclosed in the hereinafter description of a typical embodiment thereof or will be understood by those conversant with the subject. To the attainment of such objects and advantages the present invention consists in the novel method, apparatus and products herein described or claimed together with various method steps involving operations, compositions and combinations herein disclosed.

A typical prior art system of investment casting, referrcd to as an example, may be described as follows. Having predetermined and modeled the correct design for the bucket, or other part in hand, the task has been to manufacture in quantity precision castings corresponding thereto. A shaping die has first been made, as of steel, having several die parts, from which to produce so-called patterns under high pressure, these being replicas of the original design, as determined by the die, and being turned out in quantity in a conventional pressure molding machine. Such patterns were conventionally composed of a nonmetallic material such as a wax or like moldable substance of thermoplastic character so that it can be heated and poured out after the investment mold has set. The die will have operated upon the pattern in a molding press under super-pressure, even up to 50 tons per square inch, to insure a hard pattern of strong cohesion, adapted to withstand ordinary handling or mishandling and to partake of later steps in the formerly known casting systems.

Such compressed pattern in prior art practice has thus been readied for the usual investment operation, the pattern being first placed within an asbestos-lined flask or tubular box where it is mounted in preparation for embedding it within a mass of settable investment material. This material may be a conventional powdered investment compound, wetted for plasticity. The act of investing consists in first positioning the pattern within the flask or mold-box and thereupon feeding and applying investment material into the flask in a way to embed the pattern, leaving one or more access openings, passages or gates through the body of the investment material and extending from the position of the pattern to the exterior, such connections being sometimes in the form of risers, extending upwardly to an access point. They may include a spout for outflow of the heated wax from the mold cavity and later to receive the molten metal poured into the same.

Defects in prior methods, including the need of eliminating the wax before casting, and of inverting the mold to remove the wax, and of melting and pouring the metal thereafter into the empty casting cavity, are all obviated by the present invention, as will now be explained.

Ways of practicing the present invention are illustrated in the accompanying drawings, which are in general diagrammatic and wherein:

Fig. l is a perspective view of a hard body of pattern material to be converted into a solid metal turbine bucket by practice of my improved method of investment casting.

Fig. 2 shows partially in elevation and partially in vertical section the pattern body of Fig. l mounted in suitable position, as uprightly, within an outer shell or flask being filled with wetted investment material or plaster compound to form a mold.

Fig. 3 is a view like Fig. 2 showing the condition existing after the pattern body embedding material or plaster has been fully applied and has become set and hardened.

Fig. 4 is a vertical section taken centrally through the length of an oven provided for the step of baking a succession of loaded investment molds like that in Fig. 3.

Fig. 5 is a view taken in central vertical section through a high-temperature furnace, preferable of the electric induction type, with mold shifting or feeding means therefor, wherein the steps of treating the pattern body material are performed to further carry out my improved casting method.

Fig. 6 is a view like Fig. 5 showing the mechanical parts in a different position in their cycle of operation.

Fig. 7 is a sectional view of the plaster investment mold partly broken away leaving the cast product or turbine bucket still partially embedded therein.

Fig. 8 is a perspective view, corresponding generally to Fig. 1, showing a neck formation on the pattern body in Fig. l shrunk to a shorter degree of projection in the ultimate form of the casting of Fig. 7.

Fig. 9 is an elevation of the ultimate cast product shown in Figs. 7 and 8, after the neck formation of the casting has been fully removed.

Referring in outline to the investment casting method of this invention, it may be characterized as involving the preparing of a mold-forming pattern of the desired shape or design of product the same composed essentially of the identical metal of which the product is to be formed but finely powdered and cohesively pressed and compacted to the exact contours of the design, so that when the investment mold is produced, embedding the metal pattern, the latter, while occupying the mold cavity, may be heated, melted to a state of fluency and cooled to produce, before final release from the mold, the desired cast article.

Stating the method more completely as one for producing unitary cast articles or parts each being a conversion and replica of a given predetermined design or shape possessed by a precedent pattern body comprising a given casting metal or alloy; it comprises preliminarily preparing or making, for the production of each of such articles, a pattern body or mold cavity delineating member which is essentially constituted of a fine powder of the given metal, the same being compressed and rendered coherent in the shape of the desired design; positioning or mounting such pattern member within the investment space of a flask, molding enclosure or box, and introducing into such flask, so as substantially to embed or surround the metal pattern therein but leaving an access gate to the pattern or mold cavity, a suitable investment composition or material which is settable as a plaster thereby to constitute an investment mold, occupied by the pattern metal; heating the embedded metal. in situ, as by induction, to a temperature sufliciently above its melting point that the molten metal may flow freely within the mold cavity thereby at least to fill and take the form of the mold before cooling and solidifying, thereby to cast the article or part as of the given design and metal; and releasing the cast article as by opening or breaking the investment mold.

A strongly preferred embodiment of the above outlined method is one wherein, with salient improvement of the method and product, the pattern shown in Figs. 1-3 is composed not merely of the powdered casting metal,

but of this metal in mixture with a substantial but Very 0 minor proportion or percentage of a binder agent or material to promote cohesion between the powder particles of the metal and for other reasons hereinafter to appear; thereby to afford a compacted pattern having improved handling and operating resistance to disfigurement, fracture or other injury, as well as improved hardness of the surface of the pattern, and the desired true precision of the ultimate product. Along with this addition to the composition of the pattern mixture, there is preferably a coordinated addition to the basic shape thereof shown in Fig. 9, the difference being seen by comparing Fig. 1 or 2 with Fig. 9; there being a definite but approximate relation between these volumes and the proportions of the mixture used for the modified pattern. This more particular and special embodiment of the invention will hereinbelow be set forth in further detail.

It is here observed that during the melting step, while the piece is being brought to temperature and before it reaches a plastic state, the piece will expand as the mold remains relatively cool; and aside from this the cornbined bulk of the particles of powdered metal will likely have a greater extent of thermal expansion than will the mold. Hence, the choice of binder should be such as to become plastic at a relatively low temperature, imparting a degree of plasticity to the entire mass of the pattern body. The excess material due to this expansion can then expand into and through or remain in the riser space rather than crack the mold, an important consideration.

Before taking up in detail the successive steps of the broad method of the invention and particular specific embodiments thereof, and the features of apparatus and product disclosed, reference will be made to so much of the method, and the articles being produced thereby, and the apparatus made use of, as appear in the Figs. 1 to 9 of the accompanying drawings.

In Fig. l is shown the complete powdered metal pattern body or mold-forming member 10, this being a preformed intermediate product between the preceding and succeeding steps of the method. As the final shape shown is merely one of innumerable designs that may be reproduced with the present invention, it is not necessary to state fully the details and practical use of this example. The pattern body 10 comprises the permanent body portion 11 of the eventual design, this having symmetrical ribbed areas 12 and the downward permanent extension 13 in this particular shape of bucket construction. In addition to these permanent portions the pattern body is shown as having also a temporary upward projection or neck formation 15, beyond the main body, containing a needed excess of mixed powdered metal and binder and providing also a gate or riser within the investment mold to be described.

The three portions or sections 11, 13 and 15 in Fig. l constitute the pattern shape, the adjunct or upward protuberance 15 being the projecting portion which must eventually be removed to yield the final product or bucket design 30 shown in Fig. 9, identical with the desired design. As will be explained further, the volume of the adjunct 15, or at least the part thereof that enters the casting mold, is of importance in carrying out the principles of the invention. At its extreme end or top portion 16 the adjunct is provided, as an example, with a mounting device shown as a cross bar or bracket 17 extending horizontally to rest atop the casting flask.

The flask or mold-box 18, Figs. 2 and 3, may be cheaply made, as of stiff or strong paper, and after use can be discarded or burned off of the mold as partly indicated in Fig. 3. It has a closed bottom 19 and side walls 20 and a top rim 21 on which the mounting bar 17 may rest for the suspension of the pattern body 10, spaced from the flask bottom and walls.

To supply or feed material into the flask, any known feeding means may be used such as the trough 23 in Fig. 2. The inflowing investment material 24 may be a conventional wet plaster, which accumulates as a settable mass 25 as the flask fills. In Fig. 3 the plaster has set into a complete mold 26 after which if desired the flask may be stripped off and disposed of. The casting cavity 27 now is completely filled by the composition of powdered metal and binder in the pattern body 10, still in unchanged condition, and ready for the next casting step.

Between the condition in Fig. 3 and the condition in Fig. 7 melting of the contents of the mold cavity has taken place and at the place where the neck formation 15 stood there is the gate, passageway or riser space 28. Full or partial emptying of the space has taken place in Fig. 7 as the molten metal flowed interiorly of the mold before cooling into what may be termed the cast or solid metal of the product. There is shown in Figs. 7 and 8 only a residuum 29 of the aforesaid neck formation 15 of the pattern body. While the pattern body, before any meltingtakes place, occupies and extends upwardly through the cavity 27 and gate 28, by the time it has solidified after melting it has in effect shrunk due to increase in density during conversion of the powdered metal of the pattern body 10 of Figs. 1 to 3 into the solid metal of the ultimate casting of Figs. 7 and 8. The longitudinal or vertical extent of the neck formation will have decreased also in cases where the pattern contains a binder material, the elimination of the binder permitting the molten metal, before freezing, to settle down into its mold-filling condition, which extends for the full height of the body portion 11 and the bucket extension 13 of the design. Due to the volume of the added neck formation 15, an excess of metal plus binder is provided, and this important excess is so arranged and calculated that with the shrinkage of the casting within the main portion of the moldspace there will yet remain thereabove a safe margin consisting of the factor of residual excess 29 of metal as described and as indicated in Figs. 7 and 8, ensuring the filling out of the casting space corresponding to the eventual design. Fig. 7 shows the solidified casting partially freed from and partially still embedded within the mold; Fig. 8 shows the ultimate casting entirely freed from the mold. At this point the remnant 29 of excess metal, upstanding outside of and beyond the desired final design of bucket or. other part, must be removed, as by machining. When this has been accomplished, as by grinding off the excess, there results the desired final product or bucket 30, the top face 31 of which has been ground flushly to the required design.

Having outlined the method diagrams of the drawings, reference will now be made to the physical apparatus disclosures; the mechanical oven as illustrated-in Fig. 4 and the mechanical furnace as in Figs. 5 and 6. It is to be understood that while the movable parts which are in action during the performance of the method are shown without interconnection or automatic sequence these are intended however to be capable of being interconnected, timed and operated according to a predetermined sequence, corresponding generally to the method steps, and the order thereof, as elsewhere herein described.

Having in certain method diagram figures, such as Fig. 3, shown the freshly set casting mold 26, containing the pattern body 10, the mold however still requiring preliminary heating, this is done, for example, by means of an oven or baking furnace 35 preferably of refractory character, made use of for drying and hardening the mold in advance of the later heating of the embedded powdered metal pattern body. In Fig. 4 is shown a baking oven 35 in a conventional form, having end closing walls 36, opposite side walls 37 and a roof or top wall 38. To carry or feed a series of set molds 26 through such an oven the following features are provided, the speed, duration and temperature being coordinated. At the oven ends are doorways, with the entrance door 39 for example at the righthand end and exist door 40 at the lefthand end; the procession of investment casting molds 26 being advanced through the oven by means of a carrier 41, which may consist of an endless .web passing about driven pulleys 42 near the ends thereof. Any suitable heating means may be employed such as the heating unit 44, which may consist of electric conductors, or of conduits carrying superheated fluid.

The successive molds 26 may thus pass through the oven either automatically or by manual operation; and'where at the exit end the molds issue through the door 40 they may be manually or automatically shifted from the traveling carrier 41 to a support or receiving platform 46, from which in turn they may be advanced to the next apparatus element, which'may be provided with a receiving or transferring platform 47, from which in succession they are passed through the succeeding apparatus toa delivery platform 48. These platforms provide convenient means for feeding or shifting the successive molds into and subsequently out of -amelting and castingapparatus which may be an electromechanical furnace 50, shown in Figs. 5 and-6.

Thus Fig. 4- shows the baking mechanism or mechanical oven'35 operable after the formation-of each investment mold 26, but before the high temperature step is performed of melting and casting in the aforesaid mechanical casting furnace. While various types of furnace are known capable of melting a metal in a mold, the principles-of the disclosed furnace are preferred, using electric induction action directed upon the pattern 10 encased within the mold 26; with temperature and duration to afford the desiring melting, and separation of the molten metal'from other contents of the mold, preceding the cooling and solidifying of the cast product.

The illustrated casting furnace is shown as built up of stationaryand movable mechanical elements. A cylindrical casing portion 51 is shown, preferably-standing upright, and-forming a guideway which rests or isattached upon the left and right platforms 47 and 48, and within which are the longitudinally guided elements. This upper casing part is continued below the platform level as a downward extension 52, this being a lower cylinder which also, takes part in guiding the interior elements, this casing structure thus providing a shaftway 53 for the purpose, within the complete casing 51, 52.

At the top end of the shaftway is an operating chamber 55, which may be considered firstly as a heating chamber, wherein the embedded pattern is brought to and above melting temperature; and secondly as a suction chamber, as will be described, the suction therein functioning to withdraw air and moisture and to provide at the sametime the operating force for raising, or at least upholding, the interior elements of the furnace apparatus. This cham ber 55. lies within. enclosing walls 56, shown as generally cylindrical but with reduced diameter at its upper portion. itslower portion constituting an upward continuation of the upper casing cylinder 51.

By these features of furnace structure theoperating chamber 55 is enclosed above and laterally by its, own walls 56 and is closable and openable by movableelements below as will be explained. At the lower end of the downward casing extension 52 is a cross piece '57, which may or may not close the lower end of this casing section, but it is of use in carrying a central coupling or eye 58 for anchoring the low end of a spring to be referred to.

The principal interior movable part of the furnace or casting apparatus 50 is a mold elevator 59, shown inthe form of a piston, slidable within the shaftway 53 so that it can be lifted and lowered between the two positions shown respectively in Figs. 5v and 6. This elevator piston orv mold-carrier 59 is formed with a top supporting surface 60 on which the successive investment molds 26 may rest, as on a shelflduring the melting and casting operation. Preferably only a single mold is treated in each operating cycle, but the action may be multiple.

This piston-elevator member 59 serves also, when lifted. as the underneath closing element of the operating chamber 55. While no tight fit between the elevator and the casing is necessary, a reasonably close contact is desirable, to protect the chamber against undue influx or efiiux of air and to aid in applying suction in part of each cyclic operation. For this purpose there is conventionally shown a packing means 61, this example being a bent composition ring attached to the underside of the elevatorpiston and running against the inner sides of the upper and lower casing walls 51 and 52. A screw clamp 62 is shown, in the form of a disk with a threaded stem, overlying the packing ring and securing it tightly to the piston underside. This clamping portion of the piston also provides a coupling eye or anchoring means 63 for tension coil spring 64. This spring extends vertically between the anchor points or eyes 58 below, and 63 above, and thus constantly pulls downwardly upon the piston. Assisted by gravity the spring may contract and pull the piston or elevator fully down to the Fig. position, when not otherwise held up, as by a latch or the suction.

Means also is provided for extending and stretching the spring as by lifting the elevator 59 to its uppermost position shown in Fig. 6; or timed mechanism may perform this action. When in the Fig. 6 position the piston 59 constitutes the closure for the operating chamber and as well the support or movable shelf upon which rests the casting mold 26 as seen in Fig. 6. When in its lowered position the top surface 60 of the operating piston or elevator is brought substantially to the position shown in Fig. 5 wherein that top surface is registered substantially flush with, or at least easily accessible from, the exterior mold-supporting platforms 47 and 48. At the left there is shown a first access doorway 66 for entrance of molds from the platform 47 at the left, the doorway being formed by a cutaway area of the upper casing cylinder 51; while at the righthand side is a similar doorway 67 cut away to make the elevator readily accessible for the removing of each mold on to the outgoing platform 48 after the heating, melting and casting have been completed.

The heating means for melting the metal pattern within the mold 26 is shown as consisting of an induction coil 69, as with a few turns of heavy water-cooled tubing. It is understood that for high frequency induction heating the coil must be water cooled. The heating, due to eddy currents, is induced into the work-piece, and a very rapid temperature rise results. The leads to this coil are arranged to pass through insulating sleeves 70 extending in a sealed manner through the chamber wall 56 in a conventional way. Said induction coil may be such as to operate conventionally, and is to be understood as connected in an appropriate circuit to deliver the high frequency alternating current necessary to generate the melting temperature conditions required, the induction heating effect being focused or directed upon the embedded metal pattern, preferably in a way to avoid overheating of the investment mold itself. Sources of electric current, including for example a high frequency converter, and switches controlling the same, are not shown but will be understood; and the switching operations may be c0- ordinated with the elevator movements, each mold to be elevated or transferred into the coil 69 within the chamber 55 just preceding the application of the current to the coil. These operations may be under timed and coordinated control by suitable automatically operating devices.

As already indicated, the conditions of vacuum or suction and pressure within the operating chamber 55, should be under control. The normal interior pressure may be atmospheric, as it must be when the parts are positioned as in Fig. 5. During the heating and melting operations the chamber should be under suction so as to carry away various gases and the moisture therefrom. Incidentally, in the Fig. 6 position the elevator may be held upward by the existence of the suction above it, thus overcoming and stretching the spring 64 and holding the parts securely in correct position for operation, the subsequent release of the suction permitting the elevator to descend and its spring mechanism to collapse, as the parts come back to the normal or initial Fig. 5 position, which may be cushioned. During heating and melting de-gassing is rapid.

To bring about the so-described air suction and pressure conditions, there is shown, as one of many available valve means, a three-way valve 72 connected to the chamber 55. The valve has a casing 73 which is mounted by a threaded neck atop the chamber wall 56, as a means of support. Within the casing wall is shown, as an atmospheric source, a passage 74 leading from the atmosphere into the valve; and there is formed also a similar passage 75 which leads from the valve to a source of suction, as a vacuum pump, not shown. The operating part of the valve 72 consists of a rotary plug 76 containing a bent passage 77 which, in one position, shown in Fig. 5, connects the chamber with the atmospheric source, while in the Fig. 6 position the same bent passage connects the chamber with the exhausting or suction source. In order to operate the valve its rotary plug 76 is provided, at an exterior point, with a handle, as in the form of an arm 78, through the swinging of which between its extreme positions the chamber may be readily and rapidly connected either to the suction means, to remove chamber air during heating and casting, or to the atmosphere to relieve the suction and vent the chamber, after the casting is completed and the furnace parts are to be restored to normal position, for the next operation.

The first stage of the present system or method is the production of the pattern body 10 of Fig. 1. Many metals that can be powdered and mixed with a binder, can serve as the material of the pattern body such as steel, or high-refractory metals, e. g. containing tungsten, chromium, nickel and silicon; or alloys such as aluminum and magnesium alloys, of high or low melting point, according to the needs. The composition of the pattern body will preferably include also a binder material to give cohesion and an ability of the powdered metal to flow with sufficiently early relief from the expansive forces thereof to prevent cracking of the investment material. There is a wide choice of binders, with preference to a hydrocarbon, as a wax. In any case it is preferred that the binder shall have elastic properties when being heated prior to the melting and casting operation. This also helps to avoid differential expansion and fracturing of the mold. Thermoplastic binders answering the description will be suitable.

Having the completed pattern, as above described, this is next mounted in the casting box, its projecting adjunct or protuberance showing above, and the investment compound is then applied, e. g. as illustrated in Fig. 2, until the flask is substantially filled with the compound, as fur ther illustrated in Fig. 3, wherein the setting of the mold has been completed, embedding the pattern 10.

Various investment compounds may be employed, it being understood that this material is preferably in the nature of a plaster, fed into the flask, and composed of settable material, which becomes hard and then dries as indicated in Fig. 3, preferably with oven-heating and further drying and hardening according to Fig. 4. The nature of the further steps may be as indicated in the diagrams Figs. 7, 8 and 9. The actual heating and melting of the pattern have already been described in connection with the electric induction furnace apparatus of Figs. 5 and 6.

All actions and steps, it has been demonstrated, including high frequency heating, melting and de-gassing are very accurately controllable.

Referring further to the investment material or composition, many types have been developed often containing quartz, crystobalite, silica, trimidite and/or ferrolite, according to the melting temperatures and the expansion characteristics required. For the higher melting point alloys the pattern is often dipped in a close-grained refractory plaster prior to pouring the investment around it to seal the immediate surface next to the casting and to give a better finish.

There have thus been described the essential features and elements of the present invention considered in the aspect of method or system and as well the aspect of apparatus or means adapted to carry out the method, and along therewith the product or. article resulting from the practice of the method; the product including not merely the final article but intermediate products or structures through which stages the method passes. Since many matters of method, step, apparatus, combination, structure and product may be variously modified without departing from the principles involved it is not intended to limit the invention to such matters except to the extent set forth in the appended claims.

What is claimed is:

1. The method of producing a solid metallic casting by the use of mold forming investment material, which comprises the steps of, forming into a pattern body having the desired size and shape of an ultimate casting increased by that of a neck formation upstanding therefrom a compacted mixture of heat expansible powdered metal and a thermoplastic binder having a fusing point much lower than that of said metal, embedding said pattern body in flowable settable investment material substantially less expansible than said powdered metal to form therewith a unit in which the main bulk of said pattern body fills said cavity and said neck formation of said pattern body forms and fills a space in said investment material rising from said mold cavity, subjecting said embedded pattern body to increasing heat by stages operative successively to soften said binder while said powdered metal is expanding and later to melt said powdered metal, whereby said pattern body becomes sufliciently plastic prior to the melting of said powdered metal to enable components of said mixture to pass from said mold cavity into said riser space thereabove for accommodating expansive enlargement of said pattern body without subjecting said investment material to fracture by expansive force of said pattern body, cooling the molten metal to form a solid casting, and freeing said casting from the mold.

2. The method defined in claim 1, in which the volume of powdered metal in the said pattern body is sufiicient to fill at least the whole of the said mold cavity after the said molten metal has cooled.

3. A method of producing a metallic casting as defined in claim 1, wherein the said investment mold is formed by supporting the said pattern body in a mold flask and filling said flask around said article with a flowable settable investment substance.

4. A method of producing a metallic casting as defined in claim 3, wherein the said flowable settable substance is wet plaster.

5. The method defined in claim 3, in which the said mold flask is open at the top, and the said pattern body is supported in the said mold flask by suspending the said neck formation of said pattern body from a top portion of said open flask.

6. A method of investment casting for converting a pattern body of mixed expendible and permanent substances having substantially difierent melting points into a cast article of solid metal having the same shape and size as least a part of said body but composed of the permanent substance alone, which includes the steps of, compacting into a necked pattern body of self-sustaining shape and size discrete particles of a permanent metallic substance having a relatively high melting point and expansible by heat commingled with an expendible thermoplastic binder substance having a relatively low melting point, embedding said pattern body in flowable settable investment material to form a mold and delineate therein a mold cavity completely filled by said body, establishing a venting and overflow passageway through said investment material extending from said embedded body to the exterior of said mold, hardening said investment material, subjecting said embeddded body to a relatively low initial intensity of heat sufiicient to soften only said expendible binder substance thereby to promote fluidity in the commingled substances of said body while said discrete-particles of permanent substance remain unsoftened and undergo expansion responsively to said heat, whereby contemporaneously with said expansion a portion of said substances is forced out from said cavity into said passageway in sufficient volume to make room in said cavity for said expansion without build-up of disintegrating pressure against said hardened investment material, increasing said initial intensity of heat to a temperature sufficient to exude from said cavity into said vent substantially all of said expendible substance and to melt said permanent substance into a molten mass capable of flowing by gravity into full occupancy of said cavity, and cooling said molten mass when substantially devoid of said expendible substance until the former congeals into a cast article of solid metal having the same shape and size as at least a portion of said pattern body.

7. A method of investment casting as defined in claim 6, in which suction is applied to the said venting passageway simultaneously with the said heating of the said embedded pattern body thereby to promote the exuding of the said expendible substance from the said mold cavity.

8. A method of investment casting as defined in claim 6, in which the said embedded pattern body is subjected to the said heat by electrical induction.

9. A method of investment casting as defined in claim 8, in which suction is applied to the said venting passageway simultaneously with the said heating of the said pattern body.

10. The investment casting method of producing an ultimate article of solid-cast metal by conversion of a pattern body containing said metal in powered form, which includes the steps of, preliminarily preparing a necked pattern body of self-maintained predetermined shape and size composed of a compacted mixture of metallic powder with a thermoplastic binder substance having a lower specific gravity and substantially lower melting point than said powder, stationing said pattern body within otherwise vacant space in a mold flask, introducing into said flask suificient flowable and settable investment material substantially to submerse said pattern body therewithin, causmg said material to set into a rigid refractory mass forming a mold containing a cavity filled by said pattern body, initially heating said pattern body while the latter is embedded in said mold to a temperature in the softening range of said binder substance and below the melting point of said metal thereby to promote fluidity of the yet unmelted powdered metal of said body while the latter expands responsively to said initial heating, whereupon said powdered metal can settle out of said mixture and fully occupy at least the lower region of said mold cavity to the exclusion of said binder substance, subsequently heating the contents of said mold cavity to a temperature at least as high as the melting point of said powdered metal whereby the relatively heavy molten grains of said metal will coalesce and flow into full occupancy of said mold cavity thereby to force the relatively lighter binder substance upward through said cavity, cooling said molten metal to solidify the same into an article of cast metal devoid of said binder substance having the size and shape of said pattern body, and freeing said cast article from said investment material.

References Cited in the file of this patent UNITED STATES PATENTS (Other references on following page) Rose May 10, 1949 12 Skinner May 23, 1950 Brennan June 6, 1950 Brennan Nov. 21, 1950 Maddock June 5, 1951 Haller Feb. 10, 1953 FOREIGN PATENTS Great Britain of 1895 Great Britain Nov. 16, 1945 

1. THE METHOD OF PRODUCING A SOLID METALLIC CASTING BY THE USE OF MOLD FORMING INVESTMENT MATERIAL, WHICH COMPRISES THE STEPS OF, FORMING INTO A PATTERN BODY HAVING THE DESIRED SIZE AND SHAPE OF AN ULTIMATE CASTING INCREASED BY THAT OF A NECK FORMATION UPSTANDING THEREFROM A COMPACTED MIXTURE OF HEAT EXPANSIBLE POWDERED METAL AND A THERMOPLASTIC BINDER HAVING A FLUSHING POINT MUCH LOWER THAN THAT OF SAID METAL, EMBEDDING SAID PATTERN BODY IN FLOWABLE SETTABLE INVESTMENT MATERIAL SUBSTANTIALLY LESS EXPANSIBLE THAN SAID POWDERED METAL TO FORM THEREWITH A UNIT IN WHICH THE MAIN BULK OF SAID PATTERN BODY FILL SAID CAVITY AND SAID NECK FORMATION OF SAID PATTERN BODY FORMS AND FILLS A SPACE IN SAID INVESTMENT MATERIAL RISING FROM SAID MOLD CAVITY, SUBJECTING SAID EMBEDDED PATTERN BODY TO INCREASING HEAT BY STAGES OPERATIVE SUCCESSIVELY TO SOFTEN SAID BINDER WHILE SAID POWDERED METAL IS EXPANDING AND LATER TO MELT SAID POWDERED METAL, WHEREBY SAID PATTERN BODY BECOMES SUFFICIENTLY PLASTIC PRIOR TO THE MELTING OF SAID POWDERED METAL TO ENABLE COMPONENTS OF SAID MIXTURE TO PASS FROM SAID MOLD CAVITY INTO SAID RISER SPACE THEREABOVE FOR ACCOMMODATING EXPANSIVE ENLARGEMENT OF SAID PATTERN BODY WITHOUT SUBJECTING SAID INVESTMENT MATERIAL TO FRACTURE BY EXPANSION FORCE OF SAID PATTERN BODY, COOLING THE MOLTEN METAL TO FORM A SOLID CASTING, AND FREEING SAID CASTING FROM THE MOLD. 